Three-phase reactors, such as typical ebbulating bed reactors, are employed in a wide variety of catalytic, hydrotreating operations. One such operation is the hydrotreatment of poor quality residual oil to obtain usable liquid products. In this type of process, relatively large cataylst pellets are suspended in the viscous residual oil and hydrogen is introduced in excess into the bottom of the reaction vessel. The feed oil and catalysts flow upward through the reactor along with the hydrogen during the reaction.
Two essentially different types of reactions occur in such a hydrotreatment operation. First, asphaltenes, which contain a large portion of the metals, sulfur and nitrogen, are decomposed by the catalyst and agglomerate forming coke. Second, oils and resins present in the feedstock are converted to naptha and middle distillates through a largely thermal hydrocracking process.
Examples of processes for hydrotreating Atmospheric Tower Bottom residual oils are disclosed in U.S. Pat. Nos. 4,048,057 and 4,082,648. In both of these processes, the feedstock, mixed with hydrogen, contacts a cracking catalyst in a conventional moving bed reactor with a closed loop catalyst circulation system at mild hydrocracking conditions; e.g. a temperature of from about 400.degree. C. to about 450.degree. C. and a hydrogen partial pressure from about 700 psig to 3000 psig. The effluent from the reactor is subsequently cooled causing a solid asphaltic fraction to precipitate. Once precipitated, the asphaltic fraction can be removed by simple filtration or other conventional means to provide a higher quality treated feedstock for subsequent refining processes.
Conventional reactor systems employed in these processes suffer from a number of disadvantages. For example, relatively large catalyst pellets which are employed in this type of reactor provide a relatively small surface on which asphaltenes may be deposited. The asphaltenes, therefore, tend to migrate to relatively stagnant areas of the reactor where they agglomerate and form coke deposits. These coke deposits must subsequently be removed from the reactor. Additionally, the flow of gaseous hydrogen through the highly viscous fluid or slurry tends to be as large bubbles which provide little area for the mass transfer of hydrogen and products. The large amount of hydrogen which must be circulated carries off most of the more volatile and fluid components, leaving only a highly viscous residue in the reactor. A large hydrogen recovery system and recycle loop is also required.